The present invention relates to a metering device for lacquer which is incorporated into a developing apparatus for printing plates. The device comprises at least one doctor blade made of a flexible material which is detachably mounted in a guide bar connected with a blade holder, the ends of which can be inserted from the top into a channel-section member, each which is open toward the top and has two arms joined in spaced relationship with respect to one another by way of a web. On the upper side of the blade, holder adjusting means are provided at both ends.
A doctor blade assembly of this kind is disclosed in German Offenlegungsschrift No. 2,756,932 and is equipped with holding means by which the doctor blade can be attached to two side plates of a developing machine for printing plates. Each of the channel-section members of the holding means is fastened by its web to a side plate, in such a manner that its arms point toward the inside of the developing machine, transversely to the direction of transport of the printing plates. Support means are mounted at both ends on the upper side of a carrier bar which is connected with a guide bar and, via these support means, the carrier bar and the guide bar joined to it together with the doctor blade each rest on a channel-section. In this doctor blade assembly, the guide bar with the doctor blade and also the doctor blade by itself can be removed from the developing machine, quickly and without the use of tools, for cleaning or, if necessary, exchanging these components.
From the state of the art, a further doctor blade device is known, wherein the doctor blade consists of a narrow rubber strip which is cut to size and placed into a square channel-section, the arms of which are subsequently pressed together. This channel-section serves as a guide bar for the doctor blade. In addition, the doctor blade is fastened to the channel-section by screws disposed in spaced locations. The guide bar with the inserted doctor blade has only a limited inherent stability and it is therefore screwed to a blade carrier which is attached to securing elements in the machine so that a carrier bar is formed.
It is a disadvantage of this known doctor blade device that exchanging the blade is comparatively complicated and can only be accomplished with the aid of tools. For that purpose, first the carrier bar is dismantled, then the channel-section carrying the rubber strip is completely detached, after that the screws by which the rubber strip is fastened to the guide bar are removed, and finally, the rubber strip can be replaced. This work and the ensuing reassembly often cannot be performed by the user of the machine, but only by trained servicing personnel. It is particularly difficult to renew the narrow rubber strip constituting the doctor blade, because due to the softness of the rubber material, the strip deforms when it is clamped into the guide bar, especially at the screw fastening, and it is then possible that the stripping edge of the blade may no longer be straight. As a consequence, the stripping edge must often be trimmed after the rubber strip has been clamped into the guide bar.
U.S. Pat. No. 3,983,758 discloses an assembly for fastening a sponge in an automatic processor for printing plates. This assembly varies in type from the doctor blade device of the present invention, although both serve to distribute a liquid on a printing plate. The function of the sponge, however, differs from that of the doctor blade. For spreading the liquid, the sponge is adapted to perform a reciprocating motion on the printing plate, transversely to the direction of travel of the plate. On the plate-infeed side of the processor, the reciprocating sponge rubs developer solution over the printing plate. On the plate-output side, on the other hand, the sponge is used to rub washing water over the printing plate. The sponge is inserted into a U-shaped channel which opens downwardly and is supported by a rod projecting from a side plate of the processor. The projecting end of the rod is coupled with a crank drive. At the point where the sponge rod projects from the side plate, the latter is reinforced by two slide blocks each, which encompass an opening into which the sponge rod can be placed from above. The slide blocks guide the sponge rod in the direction of the reciprocating motion, transversely to the direction of travel of the printing plate.
It is not possible to simply apply this construction of the sponge assembly to a doctor blade device, since the doctor blade must be stationarily disposed in the developing machine, and the contact pressure exerted by the blade on the printing plate must be adjusted differently than the frictional pressure exerted by the sponge.
Depending upon the purpose for which a particular blade is used in a doctor blade device, it is known either to let the doctor blade contact the surface of the printing plate exerting pressure thereupon or, if a liquid is to be metered, to leave a narrow gap between the lower edge of the blade and the surface of the printing plate. In the latter case, the gap determines the thickness of the liquid layer which is to be applied by the doctor blade.
For developing and lacquering negative-working printing plates, two machines are employed. These are (1) a developing machine, wherein the developing liquid is applied to the plate, and (2) a lacquering machine, wherein the developed printing plate is coated with a lacquer layer. This processing method using two machines is time-consuming, and it is therefore unsatisfactory for practical purposes, in particular, for use in newspaper printing shops.